It is known that radio waves transmitted towards the horizon can be weakly received beyond the horizon due to an apparent reflective/diffractive nature of the troposphere. The troposphere is the layer of the earth's atmosphere from the ground to a height of approximately eight to ten kilometers (twenty-six thousand to thirty-two thousand feet). The scattering of radio waves off the troposphere, known as tropospheric scatter or troposcatter, has been utilized for commercial applications, normally on frequencies above 500 MHz for over the horizon links, and for transportable/temporary military and strategic communication systems. Troposcatter is advantageous for remote telemetry, or other links where low to medium rate data needs to be carried. Where viable, troposcatter provides a means of communication that is less costly than using satellites.
In the troposphere, the atmosphere is in continuous motion, including cloud formation and other convective effects, and there is a large decrease in temperature with height in the atmospheric layer which creates laminar atmospheric structures. Notably, there is no ionization in the troposphere layer. The turbulent motion of the air in the troposphere creates vortices, eddies, and other “blobs” as well as the laminar regions, all of which are scattering sites for radio waves. Thus, a transmitter in a tropospheric scatter system launches a high power signal, most of which passes through the atmosphere into outer space. However, a small amount of the signal is scattered when is passes through the troposphere, and passes back to earth at a distant point.
Troposcatter communication links transmit a collimated beam and receive the weakly scattered troposcatter signal beyond the horizon. Both sides of a link typically utilize the same antennas and are generally positioned to produce the same scatter angle. The scatter angle is the angle between an initial beam of radio signal propagated from a transmit antenna and the scattered beam reaching a distant receive antenna.
Collimated beams are typically created using parabolic-shaped antenna reflectors. Although the beams are initially collimated, the beams inherently spread as they propagate forward. As a result, a beam does not illuminate a single point in the troposphere, but rather a sizable volume. Beams from both sides of the link (i.e., transmit and receive beams) are pointed so as to illuminate a common volume known as the scatter volume.
By appropriately collimating and pointing the transmit and receive beams, link lengths in troposcatter communication systems from about fifty kilometers to a practical maximum of seven hundred kilometers can be achieved. The signal strength at the receive end of a troposcatter link decreases exponentially with increasing beam elevation angle and the related increase in scatter angle. Therefore, troposcatter beams are normally pointed at or close to the horizon.
Due to both long- and short-term random tropospheric irregularities, rapid variations in received power from the scatter volume can result in signal “fades” by as much as twenty or more decibels. Deep fades can occur beyond the minimum threshold of the receiver causing a loss of signal and making the use of a troposcatter communication link unreliable. To combat signal fade, diversity techniques have been utilized. These diversity techniques include, for example, spatial diversity (receiving multiple versions of the transmitted signal that have followed a different propagation path), frequency diversity (receiving multiple versions of the same signal transmitted at different carrier frequencies), polarization diversity (receiving multiple versions of a transmitted signal via antennas with different polarization), angular diversity (receiving two independent signals separated by a diversity angle), time diversity (receiving multiple versions of the same signal being transmitted at different time instances), and combinations thereof.
Spatial diversity entails transmitting the same signal with two antennas appropriately spaced and directed and using two other antennas similarly arranged for reception. The antennas at each side are typically separated by at least one hundred wavelengths to sample different scatter volumes and thereby de-correlate signal fades. At the receive end, signal processing can then reconstruct the original signal based on the signals received at both receive antennas. Unfortunately, the use of two antennas (i.e., two feeds and two reflectors) at each side of a tropospheric link is undesirably costly, complex, time consuming to set up and point the antennas, and utilizes an undesirably large footprint. It would be desirable in many troposcatter applications, particularly military and non-permanent commercial systems, to have the same or better link performance using only one transportable movable antenna at each site, rather than the two needed in a spatial diversity application.
Angular diversity entails transmitting a signal in a single beam and equipping a receiving antenna with two feed horns in close proximity to one another in such a manner that the transmitted beam is received in two different directions forming the diversity angle and giving rise to two relatively independent signals. These independent signals can be combined or otherwise processed to produce a received signal of sufficiently high intensity or signal-to-noise ratio.
Angular diversity is used less than spatial diversity due to the problem of optimizing the diversity angle, which depends on the distance between the two receiving feeds. As the diversity angle increases so does the statistical independence between the intensity fadings which appear on the two received signals, with a resulting system improvement. Unfortunately, antenna gain is simultaneously reduced because of defocusing at large diversity angles. Consequently, angular diversity with large diversity angles has only been practical with large diameter antenna reflectors (for example, greater than ten feet) in order to provide sufficient gain and other radio frequency properties.
Some attempts have been made to position two discrete feeds as close together as possible near the focal point of the antenna reflector so as to utilize angular diversity with smaller diameter antenna reflectors (for example, less than ten feed). Unfortunately, relatively high coupling loss between the antenna reflector and the feeds and other distortions result because the dual feeds must compromise their horn design in order to fit within the focal point of the antenna reflector. That is, feed assemblies should ideally have conical or corrugated feed horns. However, such large diameter conical or corrugated feed horns grossly overlap each other when positioned at the focal point of the antenna reflector. Consequently, compromises must be made in the size and shape of the feed horns that result in significant coupling losses and other issues.
Accordingly, what is needed is a feed assembly for an antenna system, such as, a tropospheric scatter communication system, that that employs angular diversity, and a dual-beam feed assembly for same that provides a high degree of isolation between beams.